1. Americans equate a lack of religiosity in general – or atheism specifically – with immorality.

2. Americans equate a lack of religiosity in general – or atheism specifically – with being un-American and/or unpatriotic.

3. There is no stigma concerning the expressed dislike of the non-religious. While there is a stigma (to varying degrees, depending on one’s social milieu) attached to being racist, or anti-Semitic, or Islamophobic, or homophobic – there has never existed a social or cultural backlash against people who openly express disdain for secular folks. So people simply feel much more comfortable expressing their dislike for atheists than, say, Latinas/os or women.

4. Insecurity on the part of the religious. Faith – believing claims without sufficient evidence, or claiming to know things that you don’t or can’t know – is an increasingly shaky endeavor. And in order for religious faith to survive, it requires a lot of social support: the more people who share it, the easier it is to maintain and reproduce. Thus, anyone who rejects the tenets of your faith, or calls them in to question, is a threat. Atheists lack a faith in God, and thus theists are particularly threatened by the growing presence of such humans, as they call into question the very thing that is ever so shaky to begin with: religious faith.